Isotopic composition of skeleton-bound organic nitrogen in reef-building symbiotic corals: A new method and proxy evaluation at Bermuda
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The skeleton-bound organic nitrogen in reef-building symbiotic corals may be a high-resolution archive of ocean nitrogen cycle dynamics and a tool for understanding coral biogeochemistry and physiological processes. However, the existing methods for measuring the isotopic composition of coral skeleton-bound organic nitrogen (hereafter, CS-delta N-15) either require too much skeleton material or have low precision, limiting the applications of this relatively new proxy. In addition, the controlling factors on CS-delta N-15 remain poorly understood: the delta N-15 of source nitrogen and the internal nitrogen cycle of the coral/zooxanthellae symbiosis may both be important. Here, we describe a new (”persulfate/denitrifier”-based) method for measuring CS-delta N-15, requiring only 5 mg of skeleton material and yielding a long-term precision better than 0.2 parts per thousand (1 sigma). Using this new method, we investigate CS-delta N-15 at Bermuda. Ten modern Diploria labyrinthiformis coral cores/colonies from 4 sampling sites were measured for CS-delta N-15. Nitrogen concentrations (nitrate + nitrite, ammonium, and dissolved organic nitrogen) and delta N-15 of plankton were also measured at these coral sites. Among the 4 sampling sites, CS-delta N-15 shows an increase with proximity to the island, from similar to 3.8 parts per thousand to similar to 6.8 parts per thousand vs. atmospheric N-2, with the northern offshore site having a CS-delta N-15 1-2 parts per thousand higher than the delta N-15 of thermocline nitrate in the surrounding Sargasso Sea. Two annually resolved CS-delta N-15 time series suggest that the offshore-inshore CS-delta N-15 gradient has persisted since at least the 1970s. Plankton delta N-15 among these 4 sites also has an inshore increase, but of only similar to 1 parts per thousand. Coral physiological change must explain the remaining (similar to 2 parts per thousand) inshore increase in CS delta N-15, and previous work points to the coral/zooxanthellae N cycle as a control on host tissue (and thus carbonate skeletal) delta N-15. The CS-delta N-15 gradient is hypothesized to result mainly from varying efficiency in the internal nitrogen recycling of the coral/zooxanthellae symbiosis. It is proposed that, in more productive inshore waters, greater food uptake by the coral causes a greater fraction of its low-delta N-15 regenerated ammonium to be excreted rather than assimilated by zooxanthellae, raising the delta N-15 of the inshore corals. If so, coral tissue- and CS-delta N-15 may prove of use to reconstruct and monitor the state of the coral/zooxanthellae symbiosis over space and time. (C) 2014 Elsevier Ltd. All rights reserved.